This invention relates to a process for improving the resistance to intergranular corrosion and cracking of perforated current collectors, in particular to the production of lead-alloy grids for use as the positive plates in a lead-acid battery. The process takes advantage of conventional processes employed to perforate flat metal sheet to form grid or mesh-type current collectors, to serve additionally as a bulk deformation treatment, which is then immediately followed by an applied heat-treatment and, optionally, subsequent quenching to yield a recrystallized microstructure in the metal which possesses significantly improved resistance to undesired corrosion and growth.
In the production of grid- or mesh-form current collectors of the kind used in electrochemical cell applications, e.g. for storage batteries, a flat metal sheet is typically perforated using a continuous process. An electrochemically active material is typically then pasted into the resulting perforated grid, the paste is flash cured using an in-line furnace and, subsequently, the continuous pasted grid structure is cut into individual battery plates.
A typical application of this conventional process is in the production of lead-acid batteries. For the purpose of simplification, the process specifically illustrated in what follows is directed to the production of lead-alloy grids for use in lead-acid batteries. However, similar methods are used in the manufacture of metallic current collectors for other electrochemical cells and batteries as well. Some of these are disclosed in the prior patents and applications described below, all of which are hereby incorporated by reference for their teachings relative to the manufacture of metallic current collectors.
Two-step processes are frequently used to prepare perforated current collectors using a continuous approach. In the first step, flat solid sheets are produced by a variety of strip casting or slab casting followed by rolling processes. The flat solid strips produced are typically wound into coils. Coils are typically stored until needed.
In the second step the strip coils are transferred to another process-line where the perforation and usually the active material pasting take place. This process generally starts with an uncoiler that unwinds the flat solid lead coils and feeds the strip into a perforation apparatus. Commonly used perforation techniques include reciprocating expanders, rotary expanders and grid punching systems. In the case of expanders the lead strip is slit and stretched to form a continuous expanded mesh. In the case punching is employed, typically rectangular coupons are punched out of the strip to create a continuous perforated grid-like sheet. The perforation processes apply mechanical stress to the lead foil by stretching and/or pressing the base material, but in all cases the overall strip deformation, as expressed by the ratio of the strip thickness before and after the perforation are kept under 10%, typically under 7.5%. The perforated strip created by any one of the listed processes travels downstream in the process to a paster, typically followed by a flash cure oven and is then cut into individual grids in tab blanker and plate dividers.
To enhance the longevity of non-consumable electrodes, current collectors and other metallic articles used in electrochemical cells, a variety of metal, metal alloys and composites have been developed. They include lead, copper, nickel, aluminum, iron, silver, zinc, lithium and their respective alloys. In many applications the environment in which the metallic articles are exposed to is highly corrosive and research is conducted to enhance the stability, e.g. by reducing the corrosion induced weight loss and growth experienced, particularly when the metallic article is exposed to oxidizing potentials and corrosive electrolytes. As many of the batteries in question are mass-produced at high speeds, continuous fabricating and processing of current collectors are becoming the manufacturing method of choice.
The prior art describes numerous methods for producing current collectors using continuous or semi-continuous processes:
Various technologies exist for producing a flat metal or metal alloy strip. They include horizontal casting processes as described by Geiger in U.S. Pat. No. 3,926,247 (1975) and Vincze in U.S. Pat. No. 5,462,109 (1995) in which a strip is cast on a chilled casting surface of a rotating drum from a pool of molten metal. Vertical casting processes, using a twin roll arrangement with an adjustable gap, are described by Folder in U.S. Pat. No. 6,003,589 (1999) and Romanowski in U.S. Pat. No. 5,518,064 (1996). Additional equipment suitable for the production of solid strips includes belt casters as described by Ashok in U.S. Pat. No. 5,131,451(1992). Numerous commercial processes exist where a slab or strip is reduced to size by sequential rollers. Prengaman in U.S. Pat. No. 3,953,244 (1976) describes stable wrought lead-calcium-tin alloy sheet which is prepared by casting, cold working the casting, preferably using rolling to one quarter of the original thickness, within two to three days after casting and heating aged work pieces sufficiently to dissolve the precipitated calcium phases.
Various processes have been described to perforate such a continuous strip to form a suitable current collector. They include reciprocating expanders such as described by Daniels in U.S. Pat. No. 3,853,626 (1974). In this process a solid ribbon or strip of lead is fed into a continuous, in-line, guillotine-type, dual expansion machine and therein expanded along its longitudinal edges to form two reticulated portions and leaving a central unexpanded portion from which a grid, header and lug are subsequently formed. The reticulated portions are uniformly stretched in a direction perpendicular to the central portion by embossed forming rolls and matched counter rolls. Finally, the reticulated portion is rolled to twist and flatten the nodes joining skeletal elements.
Tsuda in U.S. Pat. No. 3,959,016 (1976) describes a manufacturing method for a lead-antimony alloy-grid plate for batteries comprising the fabrication of a rolled lead-alloy foil, stepwise press-punching the foil to obtain a perforated plate and hardening the plate using heat-treatment. In this method, the hardening of the lead alloy is accomplished either by preparing the rolled lead-alloy plate through a hot rolling process or by carrying out a heat-treatment before or after the press-punching process to increase the hardness of the lead grid plate for the purpose of attaining an increased strength to facilitate the battery assembly and to enable a reduction of strip thickness while maintaining dimensional tolerance and strength. Tsuda goes through great effort to avoid or at least minimize any deformation of the remaining grid strands, typically induced by punching processes, and achieves his objective by employing a multi-step punching process. The rolled lead alloy plate is passed through a funnel furnace at 210 to 220xc2x0 C. for between 30 and 90 minutes, optionally water quenched, and hardened by a subsequent natural aging process over 24 hours.
Hug in U.S. Pat. No. 4,151,331 (1979) discloses a lead-acid battery grid having a network of integrally connected strands of lead in which a portion of the strands are offset and project from one face of the grid while a second portion of strands projects and are offset to the other opposed face of the grid. The perforated structure is formed from a lead foil by a punching process. Hug indicates that it is known that lead-acid battery grids are formed from lead sheets by slitting and stretching to form an-expanded mesh. He indicates that the expansion process involves considerable cold-working of the lead which leads to corrosion of the grid, and is therefore undesirable. Hug goes further stating that perforated lead-grids, in which a flat lead sheet of substantially non-distortable thickness is perforated, stamped, rolled or slit to form apertures therein defining a mesh or grid structure, experiences reduced corrosion rates, as compared to expanded grids since less cold working of the lead takes place in the forming process. Hug therefore emphasizes that minimizing deformation in the perforation processes leads to superior chemical stability in the resulting grids.
Lehockey et al. In U.S. Pat. No. 6,086,691, assigned to the owner of the present application, describes lead and lead-alloy anodes for electrowinning metals such as zinc, copper, lead, tin, nickel and manganese from sulfuric-acid solutions, whereby the electrodes are processed by at least two repetitive cycles of cold deformation in the range of 30-80% and a heat treatment of 10-30 minutes at 180 to 300xc2x0 C. to induce recrystallization and to achieve at least 50% special grain boundaries.
Palumbo in C.I.P. Ser. No. 08/835,926 (1999), also assigned to the owner of rights in the present application, describes lead and lead-alloys with enhanced creep and/or intergranular corrosion resistance, especially for lead acid batteries. The lead-alloy is subjected to at least one processing cycle comprising cold working the lead alloy to reduce the thickness thereof by a substantial amount, preferably in excess of 10% and subsequently annealing the lead-alloy for a time and temperature sufficient to effect recrystallization to substantially increase the special grain boundary fraction.
It is a teaching common to the prior art on the manufacture of current collectors that any mechanical deformation imposed during metal strip perforation/expansion has a detrimental effect on the corrosion performance of the resulting perforated current collector structure. In other words, it has been commonly understood to be a requirement for the optimization of corrosion performance that mechanical deformation at that stage of manufacture be minimized or eliminated.
However, the two above-noted commonly assigned patent applications do teach the use of substantial mechanical deformation, followed by a heat-treatment appropriate for the specific application, to recrystallize the grain structure.
The inventors of the present invention have discovered that the mechanical stress and much more limited deformation imposed on a metal strip in the usual application of a perforation process at near-ambient temperature to perforate the strip, when followed by a heat-treatment step below the melting point of the metal or metal-alloy of which the strip is composed, for up to twenty minutes, leads to the formation of a recrystallized grain structure and to the increase in special grain boundary populations to greater than 50%, leading to a substantial improvement in the corrosion and growth resistance of the perforated non-consumable electrodes and current collectors which are made. Prior to treatment according to the present invention, the population of special grain boundaries in the metal-alloy is typically about 15%.
Special grain boundaries are highly resistant to intergranular degradation processes such as corrosion and cracking and are defined on the basis of the well-established xe2x80x9cCoincidence Site Latticexe2x80x9d model of interface structure (Kronberg and Wilson, Trans. Met. Soc., AIME, 185 501 (1949), as lying within xcex94xcex8 of xcexa3 where xcexa3xe2x89xa629 and xcex94xcex8xe2x89xa615xc2x0xcexa3xe2x88x921/2 (Brandon, Acta Metall., 14,1479 (1966)).
It is a principal object of this invention to provide an economical continuous process enabling the production of a perforated structure for use as current collectors in galvanic cells, which exhibits superior anti-corrosion properties and longevity.
It is a further object of the present invention to substantially increase the special grain boundary populations (Fsp) in the web or wire sections of the perforated structure to over 50% to enhance chemical properties and corrosion performance.
With a view to achieving these objects, there is provided a method of producing a metallic current conductor for use in an electrochemical or galvanic cell which comprises perforating a solid, flat metal or metal-alloy strip, using a continuous process which results in at least a local deformation of the remaining perforated structure. The perforated, formed strip is then annealed at a temperature below the melting point of the metal-alloy to yield a recrystallized microstructure. The deformation inherent in perforating the precursor metal strip, followed by annealing and recrystallization, increases the special grain boundaries in the microstructure, to the benefit of operating properties of the final article, i.e. the metallic current collector. The best properties are obtained when conditions are applied that raise the level of special grain boundaries in the microstructure to at least 50%.